The invention generally relates to wireless local area networks (WLAN). In particular, embodiments of the invention relate to handoffs that occur as wireless mobile stations (MS) move in and out of the coverage areas of wireless access points (AP).
In IEEE 802.11 networks, basic service sets (BSS) are connected to one another via networks denoted distribution systems (DS). Multiple DSs may be interconnected by routing devices. A subnetwork is a portion of a network that shares a common address component and operates at Layers 1 and 2 (physical and data link) of the OSI (Open System Interconnection) reference model. On TCP/IP (Transmission Control Protocol/Internet Protocol) networks, subnetworks are all devices whose Internet Protocol (IP) addresses have the same prefix. Routers, and the process of routing, operate at Layer 3 (network) of the OSI reference model. In IEEE 802.11 an Extended Service Set (ESS) includes two or more BSSs that use the same service set identifier (SSID).
If the MS-AP link's quality drops below a predefined threshold, or for any other reason, a MS may decide to perform “handoff” and re-connect to the network through another AP. If the MS performs handoff between two APs that are connected to the same ESS, the handoff process is usually contained in Layer 1 (L1) and Layer 2 (L2) of the OSI reference model. If the MS performs handoff between two APs that belong to different ESSs, the handoff process may usually contain additional operations at L1 and L2 and additional operations in Layer 3 (L3).
For example, the MS may have to discard its association with its current AP and to perform an authentication and association process with the new AP. If the current AP and the new AP are coupled to different IP subnetworks, the authentication and association process may be followed by an acquisition of a new IP address. Additional delay may occur in L3 and higher layers, and may involve whatever native mobility management mechanism that is used in a particular DS, for example MIP (Mobile IP) or SIP (Session Initiation Protocol).
Due to the different processes involved and their accumulated delay, a handoff between different ESSs or a handoff between different ESSs and different IP subnetworks may result in a temporary disconnection of the MS from the network. For real-time applications, for example, voice over Internet Protocol (VoIP), it may be desirable to shorten this delay and therefore reduce the likelihood of this disconnection.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity.